A vehicle occupant detection system having load sensors and an electronic control unit (ECU) for occupant detection is proposed in JP-A-2003-196790. The load sensors are arranged at seat rails for measuring loads applied to a seat including a weight of the seat. The ECU takes the load detected by the load sensors as load data and process the data. The ECU determines a condition of the seat, for example, the seat is occupied or vacant, and the seat is occupied by an adult or a child. A result of the determination is sent to an airbag system. An airbag ECU that controls operation of an airbag determines whether a deployment of the airbag is necessary based on the result of the determination. It adjusts air pressure of the airbag when the airbag is inflated.
A power supply section for the load sensors and a power supply section for the occupant detection ECU are separately provided. As a result, the load sensors are limited in reduction of their sizes and large areas for mounting the load sensors are required. Moreover, high manufacturing costs are required.
To solve this problem, the occupant detection system may be configured such that the load sensors and the occupant detection ECU share a power supply section. More specifically, a power supply line for the load sensors may be connected to the power supply section for the occupant detection ECU. With this configuration, the load sensor can be reduced in size by the size of the power supply section.
Battery power supply lines for connecting a battery with the occupant detection ECU and other units are generally arranged around the seat rails at which the load sensors are arranged. The power supply lines for the load sensors and the battery power supply lines are easily jammed in the seat rails when the occupant slides the seat. In such a case, the power supply lines may be shorted. A voltage at the batter power supply line measures a high voltage while a voltage at the sensor power supply line measures a five-volt constant voltage. Therefore, an excessive amount of voltage may be applied to the occupant detection ECU via the sensor power supply lines due to a potential difference between the battery power supply line and the sensor power supply line.
To reduce the excessive amount of voltage, an occupant detection system 100 shown in FIG. 6 is considered. The occupant detection system 100 includes a load sensor 101 and an occupant detection ECU. A power supply section of the occupant detection ECU is connected with the load sensor 101 via a sensor power supply line L100. A diode 104 is connected in the sensor power supply line L100. The diode 104 shuts off an excess voltage applied to the occupant detecting ECU 102 via the sensor power supply line L100.
Another occupant detection system 200 shown in FIG. 7 is considered to reduce the excess voltage. The occupant detection system 200 includes the load sensor 101 and an occupant detection ECU 201. The occupant detection ECU 201 includes a zener diode 202, the first transistor 203, the second transistor 204, and the power supply section 103. A large amount of current starts flowing into the zener diode 202 when a short occurs in the circuit. As a result, the first transistor 203 turns on and the second transistor 204 turns off. The excess voltage is shut off with operation of the zener diode 203 and the first and the second transistors 203, 204.
A power supply voltage applied to the load sensor 101 is reduced by the amount of a forward voltage of the diode 104 in the occupant detection system 100 in a normal condition, that is, no short is present. A power supply voltage applied to the load sensor 101 is reduced by the amount of a turn-on saturation voltage of the second transistor in the occupant detection system 200 in the normal condition. A dynamic range of the load sensor 101 becomes narrower as the power supply voltage becomes lower. As a result, the accuracy of the load sensor 101 in measuring load is reduced.